This invention relates to optical fiber cables and, more particularly, to the coloring of buffer tubes for use in optical fiber cables.
Optical fiber cables have been used for many years to transmit information at high rates and very long distances. The transmission medium of optical fiber cables are hair-thin optical fibers protected from external forces and elements by precisely designed and manufactured cable structures. A common design for optical fiber cables, well known in the art, comprises of a central member to give support and rigidity to the optical fiber cable. Around this central member are arranged a plurality of bundles of optical fibers, each bundle individually wrapped by a buffer tube. Encasing these buffer tubes is a yarn strength member coated with a layer of superabsorbent material thereon. Finally, encasing the central member, the buffer tubes, and the yarn strength member is an optical fiber cable jacket.
Because of the vast quantity of optical fibers contained in an optical fiber cable, a color coding scheme is used to identify the buffer tubes and the individual optical fibers therein. The buffer tubes are generally uniformally colored and may comprise one of a variety of different colors including blue, orange, green, brown, slate, white, red, black, yellow, violet, rose, and aqua.
It is well known in the industry that a process of making colored buffer tubes is by mixing a buffer tube material with a color concentrate, also called a colorant, in an extruder or other high temperature and high pressure mixing device. During the mixing process, pigments contained within the base resin (also called a carrier resin) bleed into the buffer tube material. The prior art generally teaches that the buffer tube material and the base resin for the color concentrate should be the same type of material because of material incompatibility. Typically, these materials are polybutylene terephthlate (PBT), polyester elastomer, nylon, fluoropolymer, acetal resin or polycarbonate. For example, PBT is commonly utilized as both the buffer tube material and base resin for the color concentrate
However, these buffer tube materials and corresponding base resins for the color concentrates have many undesirable characteristics such as those listed below. For example, PBT has a low (e.g., 20-30%) tolerance to pigment loading. PBT is also vulnerable to hydrolytic attack which can result in degradation of the physical properties of the PBT resin. In addition, the let down ratio (i.e., ratio of buffer tube material to base resin for color concentrate) that is usually employed for coloring PBT buffer tubes is 33:1 (parts by weight) in order for the buffer tube color to be in conformance with wire and cable limits of the Munsell color scheme. Since pigment loaded PBT is more expensive than ordinary PBT buffer tube material, the low let down ratio makes buffer tubes manufactured with PBT relatively costly.
Accordingly, there exists an unsatisfied need in the industry for an improved colored buffer tube that overcomes one or more of the above-cited deficiencies.
The above needs are met and other advantages are achieved by the present invention, which comprises the use of polyolefin, preferably linear low density polyethylene (LLDPE), as the base resin for color concentrate which is added to any buffer tube material known in the art. By cross-mixing these materials, many of the difficulties and limitations of the prior art are overcome.
The present invention has many benefits over the prior art, a few of which are listed below. One benefit is the higher tolerance to pigment loading achieved by using LLDPE as the base resin for the color concentrate. Also, LLDPE resin has the properties of a high flow rate and low melting temperature resulting in better dispersion. These properties yield improved pigment release and therefore uniform coloration of the buffer tube material. Moreover, LLDPE can be mixed into most buffer tube materials known in the art. Also, LLDPE is not susceptible to degradation by heat or moisture, and does not pick up moisture even on prolonged storage. Another benefit of the present invention is the higher let down ratio of buffer tube material to colorant which can range from approximately 100:1 to approximately 500:1 (parts by weight). Material compatibility, buffer tube shrinkage, cold bend, color permanence and environmental testing of extruded buffer tubes colored using LLDPE based color concentrates have demonstrated compliance with RUS PE-90 and GR-20-CORE testing protocols. Additionally, not only are cost savings achieved because of the higher let-down ratios, but also enhanced processability, A color concentrate based on a carrier resin of such thermoplastics as PBT, nylon, polyester elastomer, fluoropolymer, acetal resin, and polycarbonate is not universal. For example, a PBT based color concentrate cannot be used in a buffer tube made of acetal resin due to difference in melt processing temperatures (PBT melts at 225-265xc2x0 C. as opposed to a processing temperature of 170-210xc2x0 C. for acetals). Any additive that is added to neat PBT resin pellets at the extrusion step is generally considered a contaminant. A desired objective of an optical fiber cable manufacturer is to introduce as few contaminants as possible in order to maintain a stable process of extrusion of buffer tubes. Therefore, LLDPE based color concentrates also offer the benefit of being able to meet the aforesaid objective, such as allowing higher let down ratios, that PBT (or any other engineering thermoplastics such as nylon, polycarbonate, acetal resin, fluropolymer which are commonly employed as buffer tube materials) based colorants do not. Given the competitive nature of the optical fiber cable industry, any cost savings achieved by using less color concentrate in the manufacture of optical fiber cables which include buffer tubes can result in a competitive advantage, and therefore, are useful and desirable,
In accordance with an aspect of the present invention, in an optical fiber cable having at least one buffer tube which encases a plurality of optical fibers, the buffer tube comprises a mixture of buffer tube material and a polyolefin resin based color concentrate. The buffer tube material may be selected from the group consisting of polybutylene terephthlate, polyester elastomer, nylon, acetal resin, fluoropolymer or polycarbonate. The polyolefin resin based color concentrate may be selected from the group consisting of linear low density polyethylene, polyethylene, polypropylene, and co-and ter- polymers of ethylene or propylene. The ratio of buffer tube material to the polyolefin resin based color concentrate is approximately between 100:1 to 500:1 (parts by weight).
In accordance with another aspect of the present invention, an optical fiber cable having at least one buffer tube encasing a plurality of optical fibers is made by a process of mixing a buffer tube material with a polyolefin resin based color concentrate and heating the buffer tube material and the polyolefin resin based color concentrate in an extruder. The buffer tube material may be selected from the group consisting of polybutylene terephthlate, polyester elastomer, nylon, acetal resin, fluoropolymer or polycarbonate. The polyolefin resin based color concentrate may be selected from the group consisting of linear low density polyethylene, polyethylene, polypropylene, and co-and ter- polymers of ethylene or propylene. The ratio of buffer tube material to the polyolefin resin based color concentrate is approximately between 100:1 to 500:1 (parts by weight).
In accordance with yet another aspect of the present invention, a method for fabricating a buffer tube for use in an optical fiber cable, comprises heating a buffer tube material and a polyolefin resin based color concentrate, and forming a buffer tube from the heated buffer tube material and the polyolefin resin based color concentrate. An additional step may comprise mixing the buffer tube material and the polyolefin resin based color concentrate. This mixing step may be performed before feeding the buffer tube material and the polyolefin resin based color concentrate into the extruder. Alternatively, the mixing step may be performed by first feeding one of the buffer tube material and polyolefin resin based color concentrate into the extruder, allowing the one of the buffer tube material and polyolefin resin based color concentrate to heat, and then feeding the other one of the buffer tube material and polyolefin resin based color concentrate into the extruder. The buffer tube material may be selected from the group consisting of polybutylene terephthlate, polyester elastomer, nylon, acetal resin, fluoropolymer or polycarbonate. The polyolefin resin based color concentrate may be selected from the group consisting of linear low density polyethylene, polyethylene, polypropylene, and co- and ter- polymers of ethylene or propylene. The ratio of buffer tube material to the polyolefin resin based color concentrate is approximately between 100:1 to 500:1 (parts by weight).